Those were the observations I made. It took some time to work what seems to be happening, but it appears to happen every time I try it on purpose.

I delete and reload Ancestry Mobile when it refuses to "talk" to Ancestry.co.uk and then all is fine as long as I only use it to look at my tree.

None of the ancestry "sync problem" article works when Mobile is involved, but if FTM fails to sync then their process of unlinking the on-line tree and downloading to FTM again with a new link is fine.